Paraffin alkylation process

ABSTRACT

A process for the alkylation of isobutane is disclosed wherein isobutane is fed to two separate alkylation systems. The effluent from the first alkylation system is fed to an interim debutanizer where the C 4 &#39;s are separated from the alkylate product. The overhead C 4  product is then fed to the second alkylation system to provide the isobutane. The effluent from the second alkylation system is fed to a traditional deisobutanizer to prevent any build up of normal butanes in the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation and claims benefit, pursuant to 35 U.S.C. §120, of U.S. patent application Ser. No. 11/402,079 filed on Apr. 11, 2006, now U.S. Pat. No. 7,449,612, which is incorporated by reference in its entirety herein.

BACKGROUND OF DISCLOSURE

1. Field of Invention

The present invention relates to the alkylation of paraffinic hydrocarbon feed stocks wherein an olefin is reacted with isobutane to produce an alkylate product. More particularly the invention relates to a process wherein an intermediate debutanizer is utilized between two alkylation systems to provide a first alkylate product and a concentrated isobutane stream for use in a second alkylation system. The process eliminates the need for a second deisobutanizer.

2. Related Information

Alkylation is the reaction of a paraffin, usually isoparaffins, with an olefin in the presence of a strong acid which produces paraffins, e.g., of higher octane number than the starting materials and which boil in range of gasoline. In petroleum refining the reaction is generally the reaction of a C₃ to C₅ olefin with isobutane.

In refining alkylations, hydrofluoric or sulfuric acid catalysts are most widely used. For sulfuric acid catalyzed alkylation low temperature or cold acid processes are favored because side reactions are minimized. In the traditional process the reaction is carried out in a reactor where the hydrocarbon reactants are dispersed into a continuous acid phase. In view of the fact that the cold acid process will continue to be the process of choice, various proposals have been made to improve and enhance the reaction and, to some extent, moderate the undesirable effects.

SUMMARY OF THE INVENTION

Briefly the present invention is an alkylation process which utilizes an interim debutanizer or stripper between at least two or more alkylation systems, preferably a process for the alkylation of isobutane with an olefin comprising a first and second alkylation systems wherein the effluent from the first alkylation system is fed to a debutanizer to produce an overhead and a first alkylation product, and wherein the overhead is fed to the second alkylation system. The capital investment benefits and energy consumption benefits, provided by the new processing scheme stem from the ability of having or producing a predominantly n-butane free olefin feedstock to be used in a first alkylation system or unit. This allows for significantly increased alkylation production without the traditional expense of additional deisobutanizer capacity, while still maintaining high quality motor-fuel alkylate production. As such, it provides refiners a new option for retrofitting existing equipment for use in expanding their facility alkylation capacity.

Two general process schemes are presented. The first general process scheme involves the use of two alkylation systems and requires an oligomerization reactor and a smaller distillation or stripping column for separation of C₄'s from heavier C₅+ materials. The first option can allow for significant reuse of equipment previously utilized for MTBE production. The second major scheme allows for processing either C₃'s or C₅'s in a first alkylation system with an interim debutanizer and processes C₄'s in a secondary alkylation system with a deisobutanizer. Several variations to the two general process schemes exist, especially for cases where alkylation of C₃ through C₅ olefin containing feedstocks is desired.

The key herein, to be able to off-load the need for additional deisobutanizer capacity, is in obtaining a nearly n-butane free olefin feed stream which may be used during an intermediate alkylation stage. This eliminates the majority of n-butane to this intermediate alkylation stage which in turn eliminates the need for an additional isobutane/n-butane fractionation step (deisobutanizer).

In one embodiment the process for the alkylation of isobutane with an olefin comprises:

-   (a) feeding a first stream containing isobutane and a second stream     containing an olefin, preferably an FCC C₄ stream containing normal     butene, isobutene, normal butane and isobutane; or a mixture of: C₄     and C₅ olefins; C₃ and C₄ olefins; or C₃, C₄ and C₅ olefins, to a     first alkylation system where a portion of the isobutane is reacted     with a portion of the olefin to form an alkylate containing stream; -   (b) feeding the effluent from the first alkylation system to a     debutanizer where C₄'s are removed as a first overheads and a first     alkylate product is removed as a first bottoms; -   (c) feeding the first overheads and a third stream containing     isobutane to a second alkylation system where isobutane is reacted     with C₄ olefins in said first overheads to form a second alkylate     product.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a simplified flow diagram of a basic C₄ oligomerization-alkylation process.

FIG. 2 is a simplified flow diagram for the alkylation of isobutane with C₄ and C₄ olefins.

FIG. 3 is a simplified flow diagram for the alkylation of isobutane with C₃, C₄ and C₅ olefins.

FIG. 4 is a plot of alkylate quality as a function of isopentane in the feed.

FIG. 5 is a plot of isopentane yield as a function of isopentane in the feed.

DETAILED DESCRIPTION

As herein defined an individual alkylation system comprises all the necessary equipment for production of a dilute alkylate stream from an olefin containing stream and an isobutane rich stream. Sulfuric acid, hydrofluoric acid and solid acid catalyst alkylation processes are contemplated. Such systems are well known in the art.

Olefin feed streams utilized herein may be hydrotreated to remove dienes prior to entering the alkylation process. The removal of butadienes and pentadienes is an important element in increasing catalyst life in the alkylation process. For processes which include oligomerization reactors to provide a purified oligomer product, the hydrotreatment step may be included (using reactive distillation) in the distillation column used to produce the oligomer stream. For feeds to oligomerization reactors, the removal of basic compounds is required to maintain catalyst life. For butylene containing streams this may include the use of a water wash column for removal of nitriles. Catalysts used in oligomerization reactors may be acid resins, such as Amberlyst 15 or related oleum derived resins and may include phosphoric acid derived catalysts, such as those known to the industry as SPA (solid phosphoric acid) catalysts.

Alkylation with C₄ Olefin Feedstock

In this case the means for obtaining the high purity olefin feed from a fluid catalytically cracked (FCC) C₄ feedstock is through the step of oligomerization, which is characterized as a process for the alkylation of isobutane with butenes contained in an C₄ stream comprising the steps of:

-   (a) feeding a first stream comprising an FCC C₄ stream containing     normal butenes and isobutenes to an oligomerization reaction wherein     the isobutenes react with each other to produce an effluent     containing C₅ and higher oligomers and normal butenes; -   (b) fractionally distilling the effluent from the oligomerization     reaction wherein the C₄ and lighter material is separated as a first     overheads from the C₅ and heavier material as a first bottoms; -   (c) feeding the first bottoms and isobutane to a first alkylation     zone to produce a first alkylate stream containing alkylate and     unreacted C₄'s; -   (d) feeding the first alkylate stream to a debutanizing zone wherein     the unreacted C₄'s are removed as a second overheads and alkylate     product is removed as a second bottoms; -   (e) feeding the first overheads and isobutane to a second alkylation     zone wherein the olefins in the first overheads are reacted with     isobutane to produce a second alkylate stream containing alkylate,     unreacted isobutane and unreacted normal butane; -   (f) feeding the second alkylate stream to a deisobutanizing zone     wherein isobutane is removed as a third overheads, alkylate is     removed as a third bottoms and normal butane is removed as a side     stream.

Optionally, the third overheads may be combined with make-up isobutane and co-fed said first stream as feed to the oligomerization reaction; and/or optionally the fractionator for separating C₄'s from the oligomers contains a bed of hydrogenation catalyst and hydrogen is fed to the fractionator such that dienes contained within said effluent are hydrogenated simultaneously with the separation. Fresh isobutane may be fed to said second alkylation zone.

FIG. 1 illustrates the overall process as it relates to the alkylation of an FCC C₄ steam with isobutane. As shown, isobutane enters the process via stream 101 and the FCC C₄'s enter the process via stream 102. The FCC C₄'s are processed prior to this step to remove any oligomerization catalyst poisons. Typically this can be accomplished by using a water wash column (not shown). The make up isobutane used for production of alkylate enters in stream 101 at a high purity, i.e. >85 vol. %. If such a stream is not available a more dilute, paraffinic, isobutane containing stream may be brought into the deisobutanizer, 23, for fractionation (not shown). The overall combined make up isobutane stream produced via the combination of streams 103 and 101 is depicted as stream 104. Stream 104, the rich isobutane stream, is then split into two streams, 105 and 106. Stream 105 is fed to a second alkylation reactor as indicated below. Stream 106 is mixed with the FCC C₄ stream. The reason for mixing stream 106 with the FCC C₄ feed stream is for use as a heat sink to handle a portion of the heat of reaction during the oligomerization step. For water-cooled oligomerization reactors it is not necessarily required and is thus optional.

The FCC C₄ in stream 102 stream and the optional isobutane rich stream in stream 106 are combined as stream 107 which is fed to the oligomerization reactor 18 containing a bed 12 of oligomerization catalyst. The isobutene in the FCC C₄ stream is oligomerized in reactor 18 to form primarily diisobutylenes and triisobutylenes along with some co-dimers leaving a C₄ cut rich in normal butenes. The effluent from the reactor 18 in stream 108 comprising mixed C₄ and oligomers is fed to distillation column 19 wherein the oligomers are separated from the unreacted C₄'s. If desired a bed 22 of hydrogenation catalyst may be utilized in conjunction with a hydrogen feed via stream 115 to simultaneously provide removal of the dienes in the C₄ stream. The removal of the dienes helps the downstream alkylation units as diene presence increases the alkylation unit catalyst consumption. The C₄'s (normal butene, and isobutane), are removed as overheads, condensed and removed via stream 109 which are later fed to a first alkylation system 24. Unreacted hydrogen is vented via stream 124.

The bottoms product oligomers from reactive distillation column 19 exit in stream 110 and are mixed with isobutane rich stream 105 to form a mixed alkylation feed stream 111 which is fed to a second alkylation system 20 wherein a portion of the isobutane reacts with olefins in stream 111 to form alkylate product. The effluent from the alkylation system 20 exits as stream 112 and contains considerable isobutane along with heavier alkylate products ranging from C₅-C₁₆. Stream 112 is sent to a stripping or distillation column 21 in which the C₄'s are removed as overheads and product alkylate #2 is removed as stream 114. The C₄'s are removed as overheads via stream 113 are condensed and combined with streams 109 and 125 to form mixed stream 127 which is fed to the first alkylation unit 24 for production of alkylate by reaction with the olefins (butenes) in stream 109.

The effluent from the alkylation system 24 is removed in stream 116 and fed to a traditional deisobutanizer where alkylate product is removed as bottoms in stream 117.

Normal butane is removed in side stream 126 and isobutane is removed as overheads as stream 103 which is recycled to the process.

The whole point of the process is to produce more high quality alkylate at a facility and prevent the necessity of additional deisobutanizer capacity, which is a considerably large fractionation tower requiring significant energy usage for separation.

Instead, for facilities having more than one alkylation system, the need for additional deisobutanizer capacity is averted by the addition of (a) an oligomerization reactor 18, (b) a fractionation tower 19 for separation of the oligomers from unreacted C₄'s and (c) a fractionation tower 21 for separation of alkylate product from the second alkylation system 20. The trade off on a piece count (numbering three new pieces of equipment) is valuable in that the replacement pieces are small in size. For instance the required distillation stages for fractionators 19 and 21 can number less than eight stages each, whereas an additional deisobutanizer column will likely require 50-80 stages. This can significantly reduce overall equipment cost as less total steel is required to obtain the same quantity of motor fuel alkylate production. The process can be utilized with any type of alkylation process, solid acid alkylation, HF alkylation, sulfuric acid alkylation and combinations thereof.

Alkylation with a Mixed C₄/C₅ Olefin Feedstock

Where C₄ and C₅ feed streams are alkylated a staged alkylation system approach can be used which is similar to that shown in FIG. 1, which is characterized as a process for the alkylation of isobutane, C₄ and C₅ olefins comprising the steps of:

-   (a) feeding isobutane and a stream containing C₅ olefins to a first     alkylation zone wherein a portion of the isobutane reacts with C₅     olefins to produce a first alkylate stream containing a first     alkylate and unreacted isobutane; -   (b) feeding said first alkylate to a debutanizing zone wherein the     first alkylate is separated as a first bottoms from a first     overheads containing the unreacted isobutane; -   (c) feeding the first overheads and a stream containing C₄ olefins     to a second alkylation zone wherein isobutane reacts with the C₄     olefins to produce a second alkylate stream containing a second     alkylate and unreacted isobutane, -   (d) feeding the second alkylate stream to a deisobutanizing zone     wherein the second alkylate is separated as a second bottoms from an     overheads containing the unreacted isobutane. Preferably any normal     butane contained in said isobutane stream passes through said first     and second alkylation systems and is removed as a side stream from     said deisobutanizer. Fresh isobutane may be fed to said second     alkylation system to make up for that which is consumed as part of     the reaction, and a portion of said first overheads may be recycled     to said first alkylation system.

Referring now to FIG. 2 alkylation feed consisting of FCC C₅'s with trace amounts of n-butane (<1 vol. %) can be used wherein an interim debutanizer 250 is utilized between two alkylation systems 230 and 240 which are processing C₅'s and C₄'s respectively. It is assumed that actual separation of the C₄ and C₅ olefins (not shown) occurs upstream of FIG. 2. The C₅ feed is sent to the first alkylation system 230 via stream 201. The feed is mixed with the combined make up isobutane from stream 209 and the overheads from the deisobutanizer in stream 215 before being fed to the first system 230 as stream 202.

The first alkylation system 230 is used to produce a dilute alkylate stream 204 containing a significant portion of isobutane as the primary constituent. The effluent in stream 204 is sent to debutanizer 250 which produces a bottoms alkylate stream 205. The debutanizer 250 can be a stripper or a full distillation column requiring reflux. The debutanizer is used instead of a traditional deisobutanizer to reduce the requirement of distillation stages and potentially the quantity of reflux and associated energy. The difference herein is that as the requirements are reduced away from a deisobutanizer operation, small quantities of isopentane, residing in the debutanizer overhead product are allowed to enter the second alkylation system 240. These minor quantities have a very minor effect on the overall alkylate quality obtained in alkylation system 240. Details of the effect are outlined below.

The resulting overhead stream 206 (containing primarily isobutane, a minor amount of n-butane, a small amount of isopentane and possible containing trace amounts of 2,4-dimethyl hexane and 2,2,4-trimethyl pentane) is fed along with either FCC C₄'s or an MTBE raffinate (containing mixed butylenes) in stream 207 to a second alkylation system 240 where they are allowed to react to produce a dilute alkylate product stream 208 in which a primary effluent constituent is isobutane. Stream 208 is then sent to a traditional deisobutanizer 260 for separation wherein an isobutane rich overhead product is removed as stream 215 and recycled to alkylation system. Alkylate product is removed as bottoms as stream 210. A normal butane rich stream is removed stream 217.

A significant variation of isobutane to olefin ratios in the two alkylation system and significant variability between alkylation for C₄ olefins versus alkylation from C₅ olefins can be achieved by utilizing flow lines 211 and 212. As one wishes to produce more alkylate from C₄ olefins, flow line 212 is utilized. To produce more alkylate product from C₅ olefins flow line 211 is used. The use of these flow lines stems from the alkylation system requirement to operate within a certain isobutane to olefin volumetric ratio, varying from as low as 4:1 to as high as 15:1. As quantities of alkylate production from individual C₄ and C₅ feeds vary (i.e. total volumes of C₄ and C₅ olefins vary), the need for higher volumes of isobutane for the different alkylation systems becomes necessary.

In a manner similar to the use of flow lines 211 and 212, a modification of the process shown in FIG. 1 may be made which adds an additional isobutane rich stream 125 to serve as a bypass around the oligomerization section (reactor 18 and column 19). This allows for better energy utilization as it reduces the duty on the distillation column 19. The inclusion of the bypass 125 depends on the overall isobutane to olefin ratio desired in the alkylation systems and the exit requirements (temperature, pressure, number phases) set for the oligomerization reactor 18 as discussed earlier.

The ability to produce two separate alkylate products from stream 205 may be desired. The alkylate product in stream 205 may be fractionated in column 270. The alkylate product in overhead stream 213 contains higher octane and Reid Vapor Pressure (RVP) than that in bottoms stream 214. For certain regions using C₅ feed stocks, this becomes a necessary step in order to produce a primary alkylate meeting RVP specifications.

Alkylation with a Mixed C₃/C₄ Olefin Feedstock

For C₃ olefin feedstocks the same process flow scheme as in FIG. 2 may be used. Therein C₃ olefin would substitute in place of the C₅ olefin feed. Propane in the C₃ feed would be taken out in a depropanizer (not shown) residing in the first alkylation system 230.

Alkylation with a Mixed C₃/C₄/C₅ Olefin Feedstock

Combinations of the basic process shown in FIGS. 1 and 2 may be utilized for alkylation of a range of C₃-C₅, C₄-C₅ and C₃-C₅, olefin containing streams. FIG. 3 provides one basic option utilizing a single debutanizer 340 for alkylate effluent streams coming from olefin C₃ and C₅ feeds which have only trace amounts of normal butane in them. The C₃ olefin stream 301 is fed to alkylation system 320 while the C₅ olefin stream 301 is fed to second alkylation system 330. Make up isobutane is fed as stream 303 and recycled isobutane from deisobutanizer 360 is added as stream 314 to make combined isobutane stream 304. Isobutane is provided to first alkylation system 320 in stream 306 and to second alkylation system 330 in stream 305. As noted both effluents 308 and 309 are fed to a single debutanizer 340 with the overhead in stream 310 being fed to a third alkylation system 340 which utilized FCC C₄'s from stream 307 to alkylate the normal butenes in stream 310. A bottoms alkylate stream is taken from debutanizer column 340 as stream 311 and deisobutanizer 360 as stream 312.

Along these lines more involved process schemes can be derived. For instance, the C₄ and C₅ FCC feed stream may be oligomerized separately. This would allow for more flexibility in how to alkylate the olefin streams which contain only trace amounts of normal butane. Additionally the step of oligomerizing the C₅ stream may be deleted with the C₅ FCC feed being fed directly to an alkylation system and then on to the debutanizer operation. Finally a single oligomerization unit may be used for both the C₄'s and C₅'s.

A process for the alkylation of isobutane C₃, C₄ and C₅ olefins is characterized as comprising the steps of:

-   (a) feeding a first stream containing isobutane to first and second     alkylation zones; -   (b) feeding a second stream containing propylene to the first     alkylation zone wherein the propylene reacts with a portion of the     isobutane to produce a alkylate stream containing a first alkylate     and unreacted isobutane; -   (c) feeding a third stream containing C₅ olefins to the second     alkylation zone wherein C₅ olefins react with isobutane to produce a     second alkylate stream containing a second alkylate and unreacted     isobutane; -   (d) feeding the first and second alkylate to a debutanizing zone     wherein the first and second alkylates are separated as a first     bottoms from unreacted propylene and unreacted isobutane as a first     overheads; -   (e) feeding the first overheads and a fourth stream containing C₄     olefins to a third alkylation zone wherein the C₄ olefins react with     a portion of the unreacted isobutane in the first overheads to     produce a third alkylate stream containing a third alkylate and     unreacted isobutane; -   (f) feeding the third alkylate stream to a deisobutanizing zone     wherein the third alkylate is separated as a second bottoms from the     unreacted isobutane as a second overheads.

Preferably any normal butane contained in said isobutane stream passes through said first, second and third alkylation systems and is removed as a side stream from said deisobutanizer and the isobutane contained within said second overheads is recycled to said first and second alkylation systems.

Staging and Energy

From a distillation perspective the invention is basic. Reducing the overhead specification to include minor amount of C₅ materials loosens the overall design requirements as compared to a traditional deisobutanizer, allowing for a substantial decrease in distillation column stages and optionally some reduction in column diameter, and condensing duty. The use of an inter-stage debutanizer between two alkylation units to reduce the need for additional deisobutanizer capacity when expanding a C₄ MTBE raffinate alkylation operation to a full FCC C₄ operation, a C₄ alkylation operation into a C₃ and C₄ operation, a C₄ operation into a C₄ and C₄ operations or a C₄ operation into a C₃/C₄/C₅ operation, has not been heretofore described in the prior art. In the end, some amount of deisobutanizer capacity is required for the entire alkylation process flow scheme so that trace amounts of normal butane found in various feed streams (isobutane make up, olefin feeds, etc.) are not allowed to build up in the system. The invention reduces the overall deisobutanizer requirement necessary as compared with the traditional use of multiple deisobutanizers as depicted in U.S. Pat. No. 5,648,586.

Fundamental calculations can be made for comparison of a traditional expansion using two deisobutanizers for C₄ and C₅ alkylation (see U.S. Pat. No. 5,648,586) versus an expansion as disclosed herein as FIG. 2. The two cases can be compared by simple distillation calculations for the effluent from the alkylation system which produces a dilute alkylate stream. A typical dilute alkylate stream is used for comparing the new scheme versus the traditional scheme for C₄ olefin feed containing isopentane and n-pentane. The dilute alkylate product composition as produced from an FCC C₅ feedstock and a recycle isobutane stream, and a dilute alkylate product produced form alkylating a C₃ olefin feed and isobutylene feed is provided in TABLE I.

Information presented by Kranz & Albright, “Alkylation of Isobutane with Pentenes Using Sulfuric Acid as a Catalyst: Chemistry and Reaction Mechanisms.” Ind. Chem. Res. 1992, 31, pp 475-481, was used as a basis for the C₃ and isobutylene (i C₄ ⁼) cases. Although isobutylene is present herein it presents the basis used to represent the diisobutylene case shown in FIG. 1. US2004/017901 teaches that due to the de-polymerization behavior of diisobutylene, oligomers of isobutylene act essentially like isobutylene on a weight basis in an alkylation system.

TABLE 1 DILUTE ALKYLATE STREAMS FROM OLEFIN FEEDS Component, wt C₅ = Alky C₃ = Alky iC₄ = Alky fraction effluent effluent effluent Propane 0.004 0.004 0.004 iC₄ 0.589 0.530 0.536 nC₄ 0.062 0.056 0.056 iC₅ 0.109 0.012 0.032 nC₅ 0.014 0.000 0.000 C₆'s 0.033 0.016 0.023 C₇'s 0.006 0.276 0.023 TMP 0.044 0.048 0.218 DMH 0.009 0.008 0.034 C₉'s 0.084 0.005 0.023 C₁₀'s 0.018 0.036 0.017 C₁₁'s 0.005 0.008 0.031 C₁₂'s 0.021 0.002 0.038

TABLE II provides the results of the comparison between the present interim debutanizer operation and a typical, prior art deisobutanizer operation for the three major feed cases: C₅ ⁼, C₃ ⁼ and iC₄ ⁼. The basis for these cases was to produce an alkylate product having less than a 5 psia RVP maximum. Distillation overhead pressure was set to 100 psia and the overall design was chosen to be at a factor of 1.2 times the minimum reflux requirement. The feed to the columns entered as a liquid at 100° F.

For each feed case two interim options are listed, each having a different control isopentane allowed in the overhead product. The “interim” stripper provides the lowest energy and staging option to meet the minimum RVP case of 5 psia, and thus allows a variable quantity of isopentane (present in the feed) to come out in the overhead product. The “interim” debutanizer case uses a maximum overhead isopentane specification of 0.25 wt % and limits C₄'s in the alkylate product to meet at minimum of a 5 psia RVP. It also requires an associated higher quantity of duty and staging to meet these specifications when compared to the stripper case.

To an extent, the “interim” cases provide the range of operation in which an interim debutanizer operation may be configured for each of the feeds given in TABLE I. As feed composition changes variations of course exist. For the most part TABLE II provides a good view of the possible options and thus the basis for selecting the appropriate process for such a flow scheme for a C₅, C₃ and isobutylene feed.

In terms of distillation staging required there is no comparison between traditional deisobutanizer requirements and the “interim” alkylation staging operations. Staging for all interim cases is always less due to the key components chosen for separation. Energy requirements for the “interim” cases with the present debutanizer operations typically require slightly higher duty as compared to traditional well designed deisobutanizer cases and with slightly lower duties required for the stripper cases. TABLE III provides a list comparing the relative staging required and relative energy require for the various “interim” options comparing the traditional deisobutanizer, interim stripper and interim debutanizer.

TABLE II DEBUTANIZER/STRIPPER/DEISOBUTANIZER COMPARISON Reboiler Duty Minimum Minimum Actual Actual Alkylate Isopentane nC5 Wt % (BTU/Hr) Reflux Ratio Stages Reflux Ratio Stages RVP(psia) Wt % in OH in OH C₅ = Feed case Deisobutanizer 3.48E+07 0.47 33 0.57 84 To debut 0.00 0.00 and depent Interim C4 Stripper 3.07E+07 — — 0 10 5   11.30 1.00 Interim debutanizer 4.18E+07 0.76 15 0.90 35 to depent 0.25 0.00 C₃ = Feed Case Deisobutanizer 3.90E+07 0.47 33 0.57 84 To debut 0.00 Interim C4 Stripper 3.30E+07 — — 0 10 2.7 0.72 Interim debutanizer 4.04E+07 0.44 11 0.53 28 3.0 0.25 iC₄ = Feed Case Deisobutanizer 3.97E+07 0.47 33 0.57 84 to debut 0.00 Interim C4 Stripper 3.35E+07 — — 0 10 2.8 1.30 Interim debutanizer 4.57E+07 0.65 13 0.78 31 3   0.25

TABLE III Relative No. Relative Duty of Stages C₅ = Feed Case Deisobutanizer 1.00 1.00 Interim C4 Stripper 0.88 0.12 Interim debutanizer 1.20 0.42 C₃ = Feed Case Deisobutanizer 1.00 1.00 Interim C4 Stripper 0.85 0.12 Interim debutanizer 1.04 0.33 iC₄ = Feed Case Deisobutanizer 1.00 1.00 Interim C4 Stripper 0.84 0.12 Interim debutanizer 1.15 0.37

From just an energy and staging requirement, the stripper case is always preferred with a choice between traditional deisobutanizer and interim debutanizer becoming a clear tradeoff between energy and staging and thus an operational cost versus initial capital cost consideration. A significant difference is that the interim designs allow some introduction of isopentane into a secondary alkylation system.

It has been found that the addition of isopentane to an alkylation system using FCC C₄ feed can cause reduced product octane if not properly controlled. As isopentane is limited to a number as low as 0.25 wt % the difference in octane numbers between feed with and without isopentane becomes immeasurable due to the available precision of typical octane tests. This allows better comparison of the options of traditional deisobutanizer with interim debutanizer on nearly equivalent terms, with the exception that the interim case provides for production of an alkylate product meeting RVP requirement.

It is anticipated that (1) with the use of more low pressure steam available for heating, (2) the use of available process streams for feed heating, and (3) the new retro fit options that are allowed due to the reduced staging requirements, a likely choice for a refiner between an interim debutanizer operation and a traditional deisobutanizer will be that of the interim debutanizer as a means to increase their overall alkylation capacity. For the case of C₃ olefin feed as shown in TABLE III the interim options are clearly beneficial, with the stripper case having 12% of the staging, 85% of the energy and only 0.73 wt % isopentane in the overhead product due to the limited make of isopentane in the first alkylation system.

For the comparison made, the deisobutanizer case does not include the total duty required for production of an alkylate product, whereas for all interim cases, except the debutanizer C₅ ⁼ case, the associated duty provided allows for production of an alkylate product. For the C₅ ⁼ case, the feed contains so much isopentane that the debutanizer bottoms stream requires the use of a depentanizer to meet the alkylate RVP requirement. Thus additional equipment is required for that case. This is also true for the traditional deisobutanizer case.

Effect of Isopentane on a Secondary Alkylation System

As pointed out above, the fundamental effect of isopentane on product quality needs to be quantified. This was accomplished by performing an experiment wherein a mixture of isobutane and isopentane was co-fed with an FCC C₄ olefin to produce an associated alkylate product. The results are shown in FIGS. 4 and 5. As shown in FIG. 4, increased isopentane in the feed causes a drop in alkylate quality as measured by the true (research+motor octane)/2 number as produced at a constant olefin space velocity, total isoparaffin/olefin ratio, temperature and mixing energy. Comparing an overhead stream in the range of around 8 wt. % iC₅ in the iC₅−i C₄ mixture (0.08 wt. fraction as illustrated in FIG. 4) to one with none, only a slight drop in quality is seen, between 0.2-0.3 octane points. Within this range such quality effects are slight in comparison to the quality effects associated with the overall alkylation system operation (OSV, 1:0 etc). Also it can be found from FIG. 5 that the cause for the reduction in octane up and (to some extent) beyond 8 wt. % (0.08 wt. fraction as illustrated in FIG. 5) is due rather to dilution of the alkylate with isopentane rather than any significant conversion of isopentane during the alkylation process. This slight drop in quality is also offset in the scheme shown herein because isopentane, which is transferred to the next stage of alkylation (possibly reducing the overall octane of the second alkylate product) can consequently provide for the lower RVP and higher quality alkylate product in the first alkylation stage, depending on the base alkylate blending properties.

A particular finding, during the measurement of the effect of isopentane on product quality (FIG. 4), is that the net isopentane consumption (using an FCC C₄ feedstock) does not occur until the iC₅/(iC₅+iC₄) weight ratio is beyond 8 wt % (0.08 wt. fraction as illustrated in FIG. 4). Indeed, contrary to a range of conditions listed in earlier studies (U.S. Pat. No. 5,583,275) it has been found that sulfuric acid catalyzed alkylation (run at low temperatures of from 25-35° F., with acid strengths between 92-98 wt %) the incorporation of isopentane into alkylate is much reduced relative to isobutane, thus preventing overall consumption at the feed conditions. It was found and plotted in FIG. 4 that an iC₅/(iC₅+iC₄) ratio of <0.1 provides for a net yield or net make of isopentane when using an FCC C₄ feedstock which is directly opposite to that disclosed in U.S. Pat. No. 5,583,275.

While the disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the attached claims. 

1. A process for the alkylation of isobutane with an olefin comprising: (a) feeding isobutane, isopentane, and an olefin to a first alkylation system where a portion of the isobutane is reacted with a portion of the olefin to form an effluent comprising a dilute alkylate stream containing unreacted components including isopentane and isobutane; (b) feeding the effluent from the first alkylation system to a debutanizer where C₄'s and at least a portion of the isopentane are removed as a first overheads, and a first alkylate product is removed as a first bottoms; (c) feeding the first overheads and a stream containing isobutane to a second alkylation system where isobutane is reacted with C₄ olefins in said first overheads to form a second alkylate product.
 2. The process according to claim 1 wherein said olefin comprises an FCC C₄ stream containing normal butene, isobutene, normal butane and isobutane.
 3. The process according to claim 1 where said olefin contains a mixture of C₄ and C₅ olefins.
 4. The process according to claim 1 wherein said olefin contains a mixture of C₃ and C₄ olefins.
 5. The process according to claim 1 wherein said olefin contains a mixture of C₃, C₄ and C₅ olefins.
 6. The process of claim 1, further comprising feeding a fourth stream containing isobutane to the second alkylation system.
 7. The process of claim 1, further comprising (f) feeding the second alkylate stream to a deisobutanizing zone wherein isobutane is removed as a second overheads, alkylate is removed as a second bottoms and normal butane is removed as a side stream.
 8. The process of claim 1, further comprising feeding at least one of an FCC C₃ and an FCC C₅ stream to the first alkylation system.
 9. The process of claim 1, wherein isopentane in the first overheads is less than about 0.25 weight percent.
 10. The process of claim 1, wherein the first and the second alkylation systems comprise a sulfuric acid catalyst for reacting the isoolefin with an olefin.
 11. A process for the alkylation of isobutane with C₄ and C₅ olefins comprising the steps of: (a) feeding isobutane, isopentane and C₅ olefins to a first alkylation zone wherein a portion of the isobutane reacts with C₅ olefins to produce a first alkylate stream containing a first alkylate, unreacted C₅ components including isopentane, and unreacted isobutane; (b) feeding said first alkylate to a debutanizing zone wherein the first alkylate is separated as a first bottoms from a first overheads containing the unreacted isobutane and at least a portion of the C₅ components including isopentane; (c) feeding the first overheads and a stream containing C₄ olefins to a second alkylation zone wherein isobutane reacts with the C₄ olefins to produce a second alkylate stream containing a second alkylate and unreacted isobutane, (d) feeding the second alkylate stream to a deisobutanizing zone wherein the second alkylate is separated as a second bottoms from an overheads containing the unreacted isobutane.
 12. The process according to claim 11 wherein any normal butane contained in said isobutane stream passes through said first and second alkylation zones and is removed as a side stream from said deisobutanizing zone.
 13. The process according to claim 11 wherein said first bottoms containing said first alkylate is fed to a fractional distillation wherein the first alkylate is separated into a third and fourth alkylate.
 14. The process according to claim 11 wherein fresh isobutane is fed to said second alkylation zone.
 15. The process according to claim 11 wherein a portion of said first overheads is recycled to said first alkylation zone.
 16. The process of claim 11, wherein isopentane in the first overheads is less than about 0.25 weight percent.
 17. The process of claim 11, wherein the first and the second alkylation zones comprise a sulfuric acid catalyst for reacting the isoolefin with an olefin.
 18. A process for the alkylation of isobutane with C₃, C₄ and C₅ olefins comprising the steps of: (a) feeding a first stream containing isobutane to first and second alkylation zones; (b) feeding a second stream containing propylene to the first alkylation zone wherein the propylene reacts with a portion of the isobutane to produce an alkylate stream containing a first alkylate and unreacted isobutane; (c) feeding a third stream containing C₅ olefins, wherein at least one of the first stream and the third stream contains isopentane, to the second alkylation zone wherein C₅ olefins react with isobutane to produce a second alkylate stream containing a second alkylate, unreacted C5 components including isopentane, and unreacted isobutane; (d) feeding the first and second alkylate to a debutanizing zone wherein the first and second alkylates are separated as a first bottoms from unreacted propylene, unreacted isobutane, and at least a portion of the C5 components including isopentane as a first overheads; (e) feeding the first overheads and fourth stream containing C₄ olefins to a third alkylation zone wherein the C₄ olefins react with a portion of the unreacted isobutane in the first overheads to produce a third alkylate stream containing a third alkylate and unreacted isobutane; (f) feeding the third alkylate stream to a deisobutanizing zone wherein the third alkylate is separated as a second bottoms from the unreacted isobutane as a second overheads.
 19. The process according to claim 18 wherein any normal butane contained in said isobutane stream passes through said first, second and third alkylation zones and is removed as a side stream from said deisobutanizing zone.
 20. The process according to claim 18 wherein the isobutane contained within said second overheads is recycled to said first and second alkylation zones.
 21. The process of claim 18, wherein isopentane in the first overheads is less than 0.25 weight percent.
 22. The process of claim 18, wherein the first, the second, and the third alkylation zones comprise a sulfuric acid catalyst for reacting the isoolefin with an olefin. 